1. Field of the Invention
This invention relates to a composite tool excellent in wear resistance as well as toughness and a process for the production of the same.
2. Description of the Prior Art
Cemented carbides are well known as a tool material excellent in wear resistance and toughness, but are unfavourably compared with tool steels because of being of more expensive and brittle. Thus, composite tools of a cemented carbide and steel have been proposed and used, for example, in which the cutting edge consists of the cemented carbide and the residual part consists of the steel. In these composite tools, both the parts are generally bonded by brazing as in the case of, for example, drills and end mills. However, these composite tools have met with the problem that the brazing strength is not sufficient and the use thereof at high temperatures is limited. Furthermore, in the case of a composite tool for making a deep hole, wherein the machining part and tool supporting part have a large length, the use of a cemented carbide alloy as only the end machining part and steel as the supporting part (shank) results in the problem that the dimensional precision is inferior due to lack in toughness in the steel part.
When the whole body of a tool is made of a cemented carbide, however, the tool is expensive and encounters the problem that the shank part is lacking in strength, resulting in a tendency of breakage. When using a cemented carbide alloy having a larger quantity of a binder metal for the shank part, the breakage resistance is so increased that it is made possible to use a cheap tungsten carbide containing somewhat more impurities, but the above described problem cannot completely be solved.
Thus, the commonly used brazing method is taken into consideration. This method is effective when the tool is loaded with a relatively small force, but in general, the joint strength is insufficient because of the limited brazing area. As a method of jointing metals each other, there is used friction welding, but in principle, this method cannot be applied to jointing of cemented carbides each other.
On the other hand, diamond compacts obtained by sintering diamond fine powder with a binder of an iron group metal under ultra-high pressure at a high temperature, have a much higher wear resistance than the prior art cemented carbides and are thus suitable for use in cutting tools, wire drawing dies and drill bits. Furthermore, BN compacts using a fine powder of high pressure type boron nitride (cubic boron nitride and wurtzite boron nitride) having a next high hardness to diamond and a less affinity to iron group metals are capable of exhibiting a more excellent performance in cutting of iron group metals having such a high hardness that the cutting is impossible by cemented carbides, and have thus been watched with keen interest.
The diamond or BN compacts have the excellent properties as a tool material, but an ultra-high pressure apparatus is required for the production thereof. Thus, their size and shape are more restricted as compared with those of the cemented carbides.
In general, a tool component is a disc-shaped composite compact A as shown in FIG. 4, which comprises diamond or BN compact 1, base 2 of cemented carbide for supporting compact 1 and intermediate joint layer 3 as described in Japanese Patent Application OPI (Kokai) No. 54278/1981. This composite compact is bonded to a steel holder or shank by brazing as it is disc-shaped or after cutting in a suitable shape, for example, to form a cutter. However, it is found that when the diamond or BN compact is heated at a higher temperature than 700.degree. C. for a certain period of time during the brazing, the properties are degraded.
Therefore, the brazing is carried out using a silver braze having a low melting point. For the general use such as cutting tools, this brazing method using a low melting point braze is sufficient if the cutting tool is used under such a light cutting condition that cutting force is relatively small, but in the case of applying the compact to a use of heavy cutting, e.g. a drill bit for drilling rocks, this brazing method is not sufficient.
In a drill bit, a number of composite compacts A as shown in FIG. 4 are mounted in a bit crown as shown in FIGS. 11 (a) and (b) and used as cutters, which is disclosed in, for example, U.S. Pat. No. 4,098,362. When drilling of rocks is carried out using a drill bit fabricated by mounting composite compacts as shown in FIG. 4 in a bit crown with a low melting point braze having a melting point of lower than 700.degree. C., relatively soft rocks such as sandstone can be drilled without problem, but in the case of drilling hard rocks, some problems are encountered that the compact cutter falls off from the brazed part and the brazed part is moved. A silver braze (e.g., JIS B Ag-1) commonly used as low melting point brazing material has a shearing strength of at most 20 kg/mm.sup.2 at room temperature, which strength is markedly lowered at high temperatures. A drill bit meets with a large fluctuation of stress due to that the stress added to the cutting edge is large and rocks are scarcely uniform. Furthermore, even if a drilling fluid such as mud is used, not only the temperature of the edge, but also that of the bit itself rise in the case of drilling a high depth formation. Depending upon the kind of a formation, mud cannot be used sometimes.
When a diamond or boron nitride compact is applied to cutters for heavy cutting of a high hardness workpiece or object, e.g. a drill bit, therefore, it is very important how to fix the compact as shown in FIG. 4 to a bit crown.